We've all seen that "Thing" on a USB cable, rumored to be a ferrite core. But, what does it do other than "Satisfy Part 15 of Title 47"? There is a whole science devoted to RFI / EMI elimination and control.
We never really saw these devices years ago in the analog world. But, now that EVERYTHING is digital, they are popping up everywhere. When your power supply was a transformer, diodes and a filter capacitor, no problem. Add a transistor regulator, still no biggie. But, big, bulky and not very efficient. Then, switching supplies became popular. 80 to 250 VAC comes in through diodes into a capacitor. This high voltage DC is fed into a transformer through a transistor switch. The transistor turns on and off at a 5kHz on up to a 1mHz rate depending on the supply. The on / off duty cycle is controlled to regulate the supplies output voltage. Since the transformer is working at high frequencies, not 60 Hz, they need no iron core. A ferrite core will do. Small, light, and over 90 percent efficient! Look at your cellphone charger. BUT, here's the problem. Switching high voltage at substantial currents at a 1mHz rate generates substantial NOISE at 1mHz, 2mHz, 3mHz, and so on right up through all our bands! 5kHz switching supplies, noise every harmonic of 5kHz! Similarly, other digital signals can produce noise as well. Here is the "Nuts and Volts" behind these mysterious bumps in the cables! A lot more than we thought!!!